On 09.08.22 22:14, Linus Torvalds wrote:
On Tue, Aug 9, 2022 at 1:07 PM David Hildenbrand david@redhat.com wrote:
/* But FOLL_FORCE has no effect on shared mappings */ if (vma->vm_flags & MAP_SHARED) return false;I'd actually rather check for MAP_MAYSHARE here, which is even stronger. Thoughts?
Hmm. Adding the test for both is basically free (all those vm_flags checks end up being a bit mask and compare), so no objections.
For some reason I though VM_SHARED and VM_MAYSHARE end up always being the same bits, and it was a mistake to make them two bits in the first place (unlike the read/write/exec bits that can are about mprotect),
But as there are two bits, I'm sure somebody ends up touching one and not the other.
So yeah, I don't see any downside to just checking both bits.
[ That said, is_cow_mapping() only checks VM_SHARED, so if they are ever different, that's a potential source of confusion ]
IIUC VM_MAYSHARE is always set in a MAP_SHARED mapping, but for file mappings we only set VM_SHARED if the file allows for writes (and we can set VM_MAYWRITE or even VM_WRITE). [don't ask me why, it's a steady source of confusion]
That's why is_cow_mapping() works even due to the weird MAP_SHARED vs. VM_MAYSHARE logic.
I'd actually only check for VM_MAYSHARE here, which implies MAP_SHARED. If someone would ever mess that up, I guess we'd be in bigger trouble.
But whatever you prefer.